1. Field of the Invention
The present invention relates to a semiconductor device or a driving method thereof.
2. Description of the Related Art
Flat panel displays such as liquid crystal displays (LCD) become widely used in recent years. However, the LCD has various drawbacks such as narrow viewing angle, narrow chromaticity range, slow response speed, and the like. Thus, as a display which overcomes those drawbacks, research of an organic EL (also referred to as an electroluminescence, an organic light-emitting diode, an OLED, or the like) display becomes active (see Patent Document 1).
However, the organic EL display has a problem that current characteristics of a transistor for controlling current which flows to an organic EL element vary by pixels. When the current flowing to an organic EL element (in other words, current flowing to a transistor) varies, luminance of the organic EL element also varies, whereby a display screen displays an image with unevenness. Thus, a method for compensating variation in threshold voltage of a transistor is examined (Patent Documents 2 to 6).
However, even if variation in threshold voltage of a transistor is compensated, variation in mobility of a transistor also leads to variation in current flowing to an organic EL element, so that image unevenness occurs. Thus, a method for compensating not only the threshold voltage of a transistor but also variation in mobility is examined (Patent Documents 7 and 8).    [Patent Document 1] Japanese Published Patent Application No. 2003-216110    [Patent Document 2] Japanese Published Patent Application No. 2003-202833    [Patent Document 3] Japanese Published Patent Application No. 2005-31630    [Patent Document 4] Japanese Published Patent Application No. 2005-345722    [Patent Document 5] Japanese Published Patent Application No. 2007-148129    [Patent Document 6] PCT International Publication No. 2006/060902    [Patent Document 7] Japanese Published Patent Application No. 2007-148128 (paragraph 0098)    [Patent Document 8] Japanese Published Patent Application No. 2007-310311 (paragraph 0026)